In the computer field, a large amount of information has been handled on a daily basis. A hard disc drive (HDD) is used as an example of an information storage apparatus for recording and reproducing such a large amount of information. The HDD is characterized by having a large storage capacity and accessing information therein at a high speed, and includes a disk-shaped magnetic recording medium and a magnetic head that records information into the magnetic recording medium.
Here, increase in recording density has always been demanded in the magnetic recording medium included in the information storage apparatus. However, in general, there are obstacles to increasing the recording density of a magnetic recording medium. One of the obstacles is so-called medium noise. Medium noise is caused by disturbance in magnetization orientation that carries information in a magnetic recording medium, and appears in a reproduction signal obtained in reproducing information from the magnetic recording medium. In order to be recorded into the magnetic recording medium, information needs to be recorded in an area large enough to provide signal components each having a sufficiently high level not to be buried in this noise (medium noise) attributable to disturbance in magnetization orientation. Accordingly, the information recording density is reduced as the medium noise level is increased.
A method for suppressing disturbance in magnetization orientation, which causes medium noise, is to increase the coercivity of a recording layer for allowing information to be recorded therein in the magnetic recording medium. The coercivity of a recording layer can be increased by reducing the grain size of the magnetic grains forming the recording layer and thus suppressing magnetic interactions among the magnetic grains. For this reason, many attempts have been made to reduce the grain size of the magnetic grains, in order to reduce the medium noise and thus to achieve high recording density of a magnetic recording medium.
For example, the following techniques have been proposed to reduce the grain size of the magnetic grains. In one of the techniques, a recording layer is formed to have a laminated structure of multiple layers. In another technique, multiple magnetic grains in a recording layer are segregated from one another by a nonmagnetic material concentrated at interfaces between the magnetic grains (grain boundaries) to form a granular structure. However, the latter technique of forming a recording layer with a granular structure often causes the following defects in the recording layer. Specifically, in this technique, some of the magnetic grains lying near the surface on which the recording layer is formed might be bonded to one another in the recording layer, which deteriorates the separation of the magnetic grains from one another in the recording layer. With this background, a technique to avoid such defects has been proposed. In this technique, under a recording layer with such a granular structure, formed is a nonmagnetic granular layer having a granular structure in which multiple nonmagnetic grains made of a nonmagnetic material are segregated from one another by another material (see Japanese Laid-open Patent Publication Nos. 2006-309919 and 2006-268972, for example).
In the technique of forming a recording layer with a laminated structure, a first layer is firstly formed as a base for constantly providing fine magnetic grains, and second and subsequent layers are formed on the first layer by growing such fine magnetic grains thereon. Meanwhile, in the technique of forming a recording layer with a granular structure, depending on the amount of a nonmagnetic material at the grain boundaries, grains of a magnetic material are segregated from one another and thus are reduced in grain size.
In this regard, the technique of forming a recording layer with a granular structure has been attracting attention because of its high effectiveness of increasing the coercivity of the recording layer while reducing the medium noise level. This effectiveness is attributed to the fact that, in this technique, not only multiple magnetic grains in the recording layer are reduced in grain size but also segregated from one another by a nonmagnetic material, so that magnetic interactions among the magnetic grains are further suppressed.
Moreover, in the technique of forming a nonmagnetic granular layer, magnetic grains in a recording layer are formed on nonmagnetic grains appearing on the top surface of the nonmagnetic granular layer, on which surface nonmagnetic grains are sufficiently separated from one another. Accordingly, this technique can provide more reliable separation of the magnetic grains from one another in the recording layer.
Recently, the demand for increasing the recording density of a magnetic recording medium has been growing more and more. In response, it has also been demanded that further increase in recording density be achieved in a magnetic recording medium using a nonmagnetic granular layer as described above by a method such as further reducing the grain size of magnetic grains in a recording layer.